In a railway freight car composed of a carbody supported by truck assemblies, side bearing assemblies on each side of the centerline of each truck bolster allow a portion of the carbody weight to be transmitted to the truck bolster at a position laterally outboard of the centerplate when the carbody leans with respect to the truck due to track irregularities, track crosslevel, or centrifugal force. A constant-contact side bearing, hereinafter referred to as a CCSB, includes a loading means to effect a supporting load at such a side bearing when the carbody has not leaned relative to the truck bolster.
In a CCSB, between the normal setup height and the solid stop height, where further leaning of the carbody is resisted by a sharply increased force, the force borne by the side bearing generally increases with increased compression of the side bearing assembly. This force is typically provided by one or more spring elements. These spring elements may be mechanical springs or elastomeric springs. At the solid stop height, another load path through very stiff elements prevents damage to the spring element. There are numerous examples in the industry, and this is well known to those skilled in the art.
A primary purpose of a CCSB is to provide a controlled resistance to truck swivel. When choosing the force which the side bearing design is to provide at the normal setup height, the carbody suspension designer must balance the need for truck hunting control at high speeds with the need for satisfactory curving behavior, especially when the car is in an unloaded condition. Normally, a higher side bearing force produces a more stable condition of the car during travel at high speed. However, if the force is too high, the increased resistance may inhibit the ability of the truck to swivel easily enough to negotiate curves, resulting in at least unnecessary wheel wear and at worst a disastrous derailment.
Association of American Railroads (AAR) standards require that the car design exhibit good stability at speeds up to 70 mph. For curve negotiation, the AAR requires that the normal setup height force be limited to no more than a value predicted by the result of a calculation found in Part B of Specification M-948 of the Manual of Standards and Recommended Practices. This calculation is specific to the car design and requires knowledge concerning the unloaded carbody weight, the distance between truck axles (wheelbase), and the restraint likely to be provided by the centerplate. Typical nominal setup height forces for different models vary between 2200 and 6000 pounds. The AAR Specification M-948 also states that the components which determine the force must have a non-interchangeability feature to prevent the inadvertent assembly of a spring element of a higher force into an assembly intended for a lower design force.
Industry requirements also dictate that the CCSB designs incorporate a method for permanently marking designs of differing forces in such a way that those persons responsible for installing the side bearing may visually determine that the correct model is being used. Additionally, maintenance personnel need to be able to see the model designation while the side bearing is assembled on the car, both to verify that the correct design is applied and, if the spring element is to be replaced, to determine whether or not the necessary replacement parts are available before the side bearing is disassembled.
CCSBs typically have a cage member attached to the truck bolster and a cap member in contact with a wear plate attached to the carbody bolster. The force of the side bearing is provided by a spring element reacting between the cage and the cap. The side bearing force is dependent upon the compression characteristics of the spring element and the design of the cage and cap. The space between truck bolster and carbody side bearing wearplate determines the normal setup height, and is achieved by shimming, ordinarily between the carbody side bearing wearplate and the carbody bolster. In order to meet the AAR requirements for marking, the model designation and nominal force at normal setup height is usually stamped or cast into the cage and/or cap members so as to be visible both before and after assembly onto the car. The non-interchangeability requirement is achieved by including physical features which assure incompatibility of the cage, cap, and spring components.
Some designs have provided for the use of a single spring element for more than one model. In this case, the basic design of the side bearing body or cap may be modified so that the compression of the spring is different for different models. This is a convenience for the user, for only one kind of spring element must be stocked to maintain several models of side bearing. A common method for executing this design is by casting or machining the floor of the cage member to different heights for different models. In this way, the appropriate force for each model is achieved by the appropriate compression at normal setup height. Examples of this method are A. Stucki Company's Compact Column Bearing™, CSB® and SSB® designs. In this case, the appropriate model designation or setup force must be cast or stamped into the member which has been chosen to be produced with variable geometry.
AAR Specifications also require bolsters to maintain a certain flatness in the area of side bearing attachment. In some cases this is requirement is not met. Particularly, the bolster mounting surface can have a “high spot,” e.g., the mounting surface under the center of the cage is higher than the “ears” (the bolt flanges on either side of the cage). Thus, when the side bearing assembly is bolted to the bolster, the cage can experience detrimental stresses and deformations that can inhibit the performance of the unit, or even result in structural failure.
In view of the above, it would be desirable make CCSBs wherein the side bearing force level could be changed, but the same cage, wear cap, and bearing element could be used. In this way, a significant cost savings can be accomplished simply by not having to make a dedicated cage for each different force level side bearing that is needed. Additionally, the CCSB could be designed to prevent the possibility of incorrect assembly while also displaying accurate model numbers or force levels in a readable manner for easy reference. It will also be desirable to provide a CCSB design that is more tolerant to non-flat bolster mounting surfaces.